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Surprisingly,
the basic principal precedes Galileo by a long time. Archimedes
(287 - 212 BC) discovered that the volume of liquid displaced by a
floating object weighs the same as the object itself. This is the
famous "Eureka" story. This principal applies also to a
submerged object that is neither rising nor sinking.
It's
easiest to imagine if you picture the glass ball as a cube of fixed
volume, with the same weight as identically sized fixed-volume
"cubes" of liquid all around it. In this steady-state
condition the glass ball is indistinguishable from the surrounding
liquid and is in equilibrium with it and neither rises or sinks. As the density of
the bulk liquid in the thermometer decreases
with an increase in temperature, these fixed volumes of liquid would
contain less liquid and as such would weigh less. In this
situation the glass ball (fixed volume & weight) is now heavier than it's surroundings and will
start to sink. Similarly, a drop in temperature would cause the ball
to rise.
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ensure that all the balls do not sink or rise together, they need to
weigh differently, so that they react at convenient temperature
intervals, normally 1 or 2°C apart. Assuming that all the balls were of
exactly the same volume (not economically possible), the weight difference between
one glass ball and the next would need to be about 1/2000th
gram. Sounds small doesn't it? Well it is - a typical
compact disc from your collection weighs just 16 grams, so the weight difference between each ball is
32,000 times less! The
lowest glass ball is the heaviest, so this sinks first as temperature
rises. The correct temperature is read from the lowest floating
ball in the top half of the thermometer.
A
detailed description of operation is supplied with every thermometer.
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